1. Field of the Invention
The present invention relates to a multiwavelength locking method and apparatus using an acousto-optic tunable filter, and more particularly to a multiwavelength locking method and apparatus using an acousto-optic tunable filter in an optical communication system including optical transport networks, in which output wavelengths of light source are monitored under the condition in which pilot signals are applied to the acousto-optic tunable filter, so as to lock the output wavelengths of the light sources, thereby eliminating an wavelength instability of the light sources for an improvement in transmission characteristics.
2. Description of the Related Art
Recently, high-speed interchange services and broadband image services have been made commercially available. As a result, an increase in the transmission capacity of communication networks has been required. Transmission of a large quantity of data has also been required in the construction of ultra-high speed communication networks, due to desires of consumers for convenience, and development of techniques and information communications. In order to meet such requirements, various methods for achieving an increase in communication speed have been proposed. Among these methods, a wavelength division multiplexing (WDM) technique adapted to simultaneously transmit different wavelengths through a signal optical fiber is being most actively studied because it allows communication over broad bandwidths.
Where such a WDM transmission scheme is applied to an optical communication system, channels are arranged at a certain wavelength interval, and signals are carried through those channels. These channels are transmitted through a single optical fiber after being optically multiplexed. In order to stabilize power intensities of diverse light sources used in this system, thereby achieving an improvement in transmission characteristics, locking of multiwavelengths is performed. As a method for simultaneously locking wavelengths of diverse light sources, a method adapted to directly apply a pilot signal to light sources has been mainly used. On the other hand, as a method for locking the wavelength of a single light source, wavelength locking techniques using a filter having a wavelength dependency such as a Fabry-Perot filter, a fiber Bragg grating, or an arrayed waveguide grating have been mainly used.
Now, a typical wavelength locking method for locking a wavelength of a single light source having a single wavelength will be described in brief.
First, beam outputted from the light source is split into two beams. And then each of the beams is passed through optical elements having different transmittances each other according to wavelength. In this case, when two optical elements which have different transmitting wavelength characteristics in terms of a transmitting wavelength peak value or the gradient of a variation in transmittance depending on a variation in wavelength are used, the ratio between the intensities of the beams passed through the two optical elements is varied each other depending on a variation in wavelength. Accordingly, wavelength locking can be achieved by controlling two light beams to have the same intensity or a desired ratio, thereby causing the light beam emitted from the light source to have a constant wavelength. (U.S. Pat. No. 6,094,446. 2000: D. Tei, et al., “Wavelength Stabilization Apparatus of Laser Source”).
Meanwhile, dense wavelength division multiplexing (DWDM) systems use a method for multiplexing beams outputted from light sources having different wavelengths distributed at a certain wavelength interval, and transmitting the multiplexed light beams. In this case, locking of the wavelength of each light source causes an increase in costs and an increase in system size because a number of light sources are used. In order to solve this problem, active research efforts have been made to simultaneously lock a number of different wavelengths. As a method for simultaneously locking diverse wavelengths, a method using pilot signals has been mainly used. In accordance with this method, pilot signals of different frequencies are applied to each of the light sources. An output signal having diverse multiplexed wavelengths is then observed at respective frequencies of the pilot signals. Thus, output characteristics of each of the light sources can be determined. For example, pilot signals having frequencies of f1, f2, . . . , and fn are applied to each of the light sources 1, 2, . . . , and n. Using a photo-detector, an optical signal obtained by multiplexing the output signals from the light sources 1, 2, . . . , and n by a DWDM system is detected at respective frequencies of the pilot signals. Based on the detected results, each of the wavelengths of the light sources can be locked in accordance with the wavelength locking method for a single light source. However, the wavelength locking method using pilot signals has a problem in that errors may be generated in optical signals because a variation in the power intensity of each light source may occur. (U.S. Pat. No. 6,118,562. 2000: H. J. Lee, et al., “Wavelength Aligning Apparatus Using Arrayed Wavelength Grating”).
Meanwhile, an acousto-optic frequency converter has been developed which utilizes characteristics of an optical signal varying in wavelength or frequency when acoustic waves interfere with the optical signal. Such an acousto-optic frequency converter is used as an optical modulator or a wavelength filter. In the case of an acousto-optic wavelength filter using optical fibers, a variation in transmitting wavelength occurs depending on a variation in the frequency of acoustic waves applied to the filter. That is, wavelength shift occurs in proportion to a variation in frequency. For this reason, it has been required to develop a method for simultaneously locking multiwavelengths using an acousto-optic wavelength filter exhibiting a variation in transmitting wavelength and a variation in frequency conversion depending on a variation in the frequency of acoustic waves applied thereto.